Volume control arrangement



April-13, 3 w. J. OBRIEN 2,077,126. VOLUME CONTROL ARRANGEMENT O Filed Nov. 9, 1934 11 12 5 5 14 13 .5 6 15. f l 1L A 7 i; 4/ 1 3 1 1 1 i2 78 1 19 7 2 1i 1/ 1 [M [0 1 01/7465 40m INVENTOR W.J.O 'RIEN BY HBm ATTORNEY Patented Apr. 13, 1937 VOLUME CONTROL ARRANGEMENT William Joseph OBrien, Chicago, Ill, assignor to Radio Corporation of America, a corporation of Delaware Application November 9, 1934, Serial No. 752,274

8 Claims (o1. 179-171) When screen grid tubes are used in a multistage amplifier, it sometimes happens that it is desirable to apply 'difierent potentials to the screen grids of the several tubes. In such an amplifier, even if similar tubes are used, and the same bias voltage is applied to the control grids, the transconductances of the tubes are different. If now the control grid bias is made more negative to reduce the gain, one or more of the tubes 0 may reach the cut-off point while another tube is amplifying in a normal manner. This condition will result in distortion of the signal. It is an object of my invention to prevent distortio due to the cause mentioned.

It is a further object of my invention to provide an amplifier circuit arrangement in which distortion is substantially eliminated in a plural stage screen grid amplifier.

A still further object of my invention is to provide a novel form of volume control for a screen grid amplifying arrangement and more especially one in which the tubes operate under conditions such that they have different transconductances.

Other objects will become apparent to those skilled in the art as the description of the invention proceeds. For a more complete understanding of the invention reference is made to the accompanying drawing, in which:

Figure l is a schematic circuit diagram of an amplifier embodying the invention; and

- Figure 2 shows a portion of the grid voltageplate current characteristic curves of two of the tubes of Figure 1.

In the amplifier of Figurel three screen grid tubes I, 2 and 3 are shown each having the usual cathode 4, control grid 5, screen grid 6 and plate or anode I. For the purpose of clarity the oathode heaters are not shown. The tuned input circuit 8 of tube I may be transformer coupled as shown to any desired source of modulated radio frequency signals such as the antenna circuit 9,

the grid 5 being connected to a point of ground potential by a lead It). The plate I of tube 1 is shown coupled to the grid of tube 2 by a transformer II, whose secondary forms a portion of the tuned grid circuit i2. The plate I of tube 2 is shown coupled to the tuned grid circuit it of tube 3 by a transformer l4 and the plate I of tube 3 may be coupled to the output terminals l 5 of the amplifier by transformer l6.

As shown the cathodes 4 are connected together to have substantially the same potential by a lead I1 and the plates connected to the positive terminal of any suitable source of direct current potential I8 by a lead IS." A resistor 22 is shown connected across theterminals of potential source !8 and having the tap points 20 and 2|. The screen grids of tubes 1 and Bare connected to point 2| by leads 23 and 24, while the screen grid of tube 2 is connected to point 20 by a lead 25, so that the grid 6 of tube 2 is maintained at a substantially lower potential than the screen grids of tubes l and 3. By-pass condensers 2B for radio frequency currents are connected between, leads 24 and 25 and ground. The control grid 5 of tube 3 is connected to ground by a lead 21, so that the control grids of tubes l and 3 are at the same direct current potential- A resistor 28 has one end connected to the cathode of tube l; or to lead l1 and its other end connected to ground, this resistor having an intermediate tap point 29 to which the control grid 5 of tube 2 is connected by a lead 32.

One end of a manual volume control resistor 30 is connected to the lower end of resistor 28 and a slidable contact or adjustable contact arm- 3| adjustable along resistor 30 is connectedby lead 32' to cathode A or lead l'l.

It will be understood'that the grid circuits 8,

l2 and I3 may each be tuned to the desired incoming signal frequency by means of the variable condensers shown or by other means known to those skilled in the art.

It will be noted that if contact 3! is slid past the left end of 30, resistor 28 is commonto the cathode and plate circuits of the three tubes the direct plate currents of all three passing through it. It will be further noted that the-control grid of tube 2 is at a higher potential than the control grids of tubes I and 3, in other words, the grid bias voltage of tube 2 is less negative than that of tubes l and 3. As contact 3| is adjusted toward the right along resistor some of the plate current will pass therethrough, less passing through 28 and the potential of point 29 will increase, or the negative bias voltage of grid 5 of tube 2 will decrease. At the same time the negative bias voltage of tubes land 3 will be volume control arrangement described, reference is made to Figure 2 in which curve 4I represents the control grid voltage-plate current characteristic of tube I and curve 42 that of tube 2. If now the control grids of these tubes had the same bias voltage, for example 5 volts, upon application of signal voltage tube I would operate about point e while tube 2 would operate about point c. If now the signal voltage is sufiiciently strong to swing the grids 5 volts, tube I would be operating on the portion a e of its curve which is substantially linear, whereas the tube 2 would be operating between point e and the cut-01f point. In this lower region of the curve as there is very small or no change of plate current with grid voltage, distortion of the signal will result. My arrangement therefore obviates this difiiculty by providing a larger negative bias for tubes I and 3. As an example, tube I may have an initial grid bias of 10 volts so that it operates about point a while tube 2 may have a grid bias of 5 volts so that it operates about point 0. Under these conditions the cut off points of both tubes will be reached at the same time, as the volume control is adjusted to decrease the gain of both tubes when it is desired to tune in a strong local station or for any other reason. Furthermore, as contact 3| is adjusted, the grid biases of tubes I and 2 are automatically changed by proportional amounts so that each tube will reach its cut-oiT point at the same time,

While the current passing through resistor 3a is here shown as controlled by a manual volume control device, it may also be controlled automatically by an automatic volume control circuit arrangement of a type well known to those skilled in the art.

Having described my invention, what I claim as novel and desire to secure by Letters Patent is:

1. An amplifier comprising a plurality of screen grid tubes, each of said tubes having a cathode, a control grid, a screen grid and an anode, means for applying a positive potential to one of said screen grids, means for applying a lesser positive potential to another of said screen grids and means for applying such a negative bias voltage to the control grids of each of said tubes that each tube reaches the cut-off point at substantially the same time.

2. An amplifier comprising at least two thermionic tubes, each tube having a cathode, a control grid, a screen grid and an anode, means for applying a positive potential to said anodes, means for applying a positive potential to the screen grid of the first tube, means for applying a negative bias to the control grid of said tube, means for applying a positive potential to the screen grid of the second tube of lesser value than that applied to the screen grid of the first tube and means for maintaining the control grid of said second tube at a more positive potential than that of said first tube.

3. The combination defined in the preceding claim in which a single control means is provided to simultaneously adjust the bias voltage of said control grids.

4. In an amplifier, the combination of at least two screen grid tubes each having a cathode, a control grid, a screen grid and a plate, a source of direct current voltage, connections between the plates of said tubes and the positive terminal of said source, a circuit connected between the positive terminal of said source and said screen grids, a fixed resistor common to the plate circuits of said tubes one end of said resistor being directly connected to the cathodes of said tubes, a connection between one of said control grids and the low potential end of said resistor and a connection between the other of said control grids and an intermediate point of said resistor.

5. The device of the preceding claim in combination with an adjustable resistor shunted across said fixed resistor, whereby adjustment of said adjustable resistor regulates the bias voltages of said control grids by proportional amounts.

6. In an amplifier, the combination of at least two thermionic tubes each having a cathode, a control grid, a screen grid and a plate, a resistor, a source of direct current connected to said resistor, a connection between the positive end of said resistor and the plates of said tubes, a connection between the screen grid of the first tube and an intermediate point of said resistor, a connection between the screen grid of the second tube and another intermediate point of said resistor which is at a lower potential than said first named intermediate point, a lead connecting the cathodes of said tubes, a second resistor having one end connected to said lead and its other end connected to the negative end of said first named resistor, a connection between the control grid of the first tube and the negative end of said second resistor and a connection between the control grid of said second tube and an intermediate point of said second resistor.

7. The combination of the preceding claim in which the intermediate point of said second resistor is so chosen that upon application of a large signal voltage to the control grid of the first tube, the plate current of both tubes is cut off at substantially the same time.

8. In an amplifier, the combination of two screen grid tubes each having a cathode, a control grid, a screen grid and a plate, each of said tubes having an input circuit connecting its cathode and control grid and an output circuit, means for impressing substantially the same positive voltage on the plates of said tubes, means for impressing positive potentials of different values on said screen grids, a resistor common to .1

the input and output circuits of said tubes, a direct current connection between one of said control grids and one end of said resistor, a direct current connection between the other of said control grids and an intermediate point of said re-- 

